Batteries are known devices that are used to store and release electrical energy for a variety of uses. In order to produce electrical energy, batteries typically convert chemical energy directly into electrical energy. Generally, a single battery includes one or more galvanic cells, wherein each of the cells is made of two half-cells that are electrically isolated except through an external circuit. During discharge, electrochemical reduction occurs at the cell's positive electrode, while electrochemical oxidation occurs at the cell's negative electrode. While the positive electrode and the negative electrode in the cell do not physically touch each other, they are generally chemically connected by at least one (or more) ionically conductive and electrically insulative electrolyte(s), which can either be in a solid or a liquid state, or in combination. When an external circuit, or a load, is connected to a terminal that is connected to the negative electrode and to a terminal that is connected to the positive electrode, the battery drives electrons through the external circuit, while ions migrate through the electrolyte.
Batteries can be classified in a variety of manners. For example, batteries that are completely discharged only once are often referred to as primary batteries or primary cells. In contrast, batteries that can be discharged and recharged more than once are often referred to as secondary batteries or secondary cells. The ability of a cell or battery to be charged and discharged multiple times depends on the Faradaic efficiency of each charge and discharge cycle.
While rechargeable batteries based on sodium can comprise a variety of materials and designs, most, if not all, sodium batteries requiring a high Faradaic efficiency employ a solid primary electrolyte separator, such as a solid ceramic primary electrolyte membrane. The principal advantage of using a solid ceramic primary electrolyte membrane is that the Faradaic efficiency of the resulting cell approaches 100%. Indeed, in almost all other cell designs electrode solutions in the cell are able to intermix over time and, thereby, cause a drop in Faradaic efficiency and loss of battery capacity.
The primary electrolyte separators used in sodium batteries that require a high Faradaic efficiency often consist of ionically conducting polymers, porous materials infiltrated with ionically conducting liquids or gels, or dense ceramics. In this regard, most, if not all, rechargeable sodium batteries that are presently available for commercial applications comprise a molten sodium metal negative electrode, a sodium β″-alumina ceramic electrolyte separator, and a molten positive electrode. One known molten positive electrode is molten NiCl2, NaCl, and NaAlCl4, commonly called a ZEBRA cell. The ZEBRA cell normally operates in a temperature range from 270° C. to 350° C. Another known molten positive electrode is a composite of molten sulfur and carbon, commonly called a sodium/sulfur cell.
Because these conventional high temperature sodium-based rechargeable batteries have relatively high specific energy densities and only modest power densities, such rechargeable batteries are typically used in certain specialized applications that require high specific energy densities where high power densities are typically not encountered, such as in stationary storage and uninterruptable power supplies.
Despite the beneficial characteristics associated with some conventional sodium-based rechargeable batteries, such batteries may have significant shortcomings. In one example, because the sodium β″-alumina ceramic electrolyte separator is typically more conductive and is better wetted by molten sodium at a temperature in excess of about 270° C. and/or because the molten positive electrode typically requires relatively high temperatures (e.g., temperatures above about 180° C.) to remain molten, many conventional sodium-based rechargeable batteries operate at temperatures higher than about 270° C. and are subject to significant thermal management problems and thermal sealing issues. For example, some sodium-based rechargeable batteries may have difficulty dissipating heat from the batteries or maintaining the negative electrode and the positive electrode at the relatively high operating temperatures. In another example, the relatively high operating temperatures of some sodium-based batteries can create significant safety issues. In still another example, the relatively high operating temperatures of some sodium-based batteries require their components to be resistant to, and operable at, such high temperatures. Accordingly, such components can be relatively expensive. In yet another example, because it may require a relatively large amount of energy to heat some conventional sodium-based batteries to the relatively high operating temperatures, such batteries can be expensive to operate and energy inefficient.
Thus, while molten sodium-based rechargeable batteries are available, challenges with such batteries also exist, including those previously mentioned. Accordingly, it would be an improvement in the art to augment or even replace certain conventional high temperature molten sodium-based rechargeable batteries with other molten sodium-based rechargeable batteries operable at temperatures below about 220° C., and more preferably below about 180° C.